Dielectric gaseous mixture of thiazyltrifluoride and sf6



R. E. EIBECK June 25, 1968 DIELECTRIC GASEOUS MIXTURE OF THIAZYLTRIFLUORIDE AND Slb` Filed Dec. 27, 1965 @um .fzwomma ZOEmOaEOO f2 :om

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RICHARD E. EIBECK BWM/fw? AGE/VT United States Patent O 3,390,091 DIELECTRIC GASEOUS MIXTURE OF THIAZYLTRIFLUORIDE AND SFS Richard E. Eibeck, Convent Station, NJ., assgnor to Allied Chemical Corporation, New York, N.Y., a corporation of New York Filed Dec. 27, 1965, Ser. No. 516,449 3 Claims. (Cl. 252-635) ABSTRACT OF THE DISCLOSURE Mixtures of thiazyltriuoride and sulfur hexauorde have been found to possess dielectric properties which render them useful as gaseous electrical insulating media, such mixtures being superior to pure sulfur hexauoride. These mixtures preferably contain from about l to about 80% by volume of thiazyltriuoride and correspondingly 20-90% by volume ysulfur hexauoride.

This invention relates to mixtures of thiazyltrifluoride and sulfur hexauoride, useful as gaseous electrical insulating media.

It is known to use sulfur hexauoride as a gaseous dielectric medium for sealed electrical apparatus such as, for example, dry type and vapor cooled type transformers, capacitors, switchgear, reactors, cables and other forms of electrical apparatus contained in sealed housings. However, it has been found that the electrical potentials which can be employed in electrical apparatus using sulfur hexafluoride as a dielectric medium are limited by the electrical breakdown strength of the sulfur hexauoride, particularly when it is used in nonuniform fields, i.e., electrical fields wherein there is a non-proportional distribution of electrical potential in the space between the electrodes. Non-uniform fields exist, for example, between electrically charged, spaced surfaces having different shapes, such as a ball and a plane, or -a point and a plane.

There is a need in the art for gaseous dielectric media having dielectric strength superior to sulfur hexaiiuoride, particularly in non-uniform field applications. Provision of dielectric media having increased dielectric strength allows charged surfaces of electrical apparatus to be spaced closer together, thus giving rise to smaller and lighter apparatus. Provision of media having increased dielectric strength also makes it unnecessary to pressurize sulfur hexaiiuoride to achieve -an increase in dielectric strength. Elimination of the need for pressurization is greatly advantageous, due to the obvious simpliication of apparatus which it makes possible.

In accordance with the present invention, it has been discovered that gaseous dielectric media comprising mixtures of thiazyltriuoride and sulfur hexauoride provide resistance to electrical breakdown which is superior to that of sulfur hexatluoride alone under comparable conditions. Preferably, the dielectric media of this invention comprise mixtures of to 80% by volume of thiazyltriuoride and, correspondingly, to 90% by volume of sulfur hexafluoride.

Sulfur hexaiiuoride is a gas that solidilies directly from its gaseous state at atmospheric pressure at about 64 C. Thiazyltrifl'uoride, NSF3, is a gas that liqueies at about 27.1 C. at atmospheric pressure. Both gases and their methods of preparation are known.

Por a better understanding of the nature of this inven- 3,390,091 Patented June 25, 1968 tion, reference should be made to the following detailed description and to the accompanying drawing.

Breakdown measurements were made for various thiazyltrifluoride-sulfur hexauoride mixtures in a two-inch diameter hollow gas-tight cell having a volume of about 0.4 liter. The cell was provided with means allowing introduction gases and means for measuring internal pressure. Inside the cell were electrodes consisting of a diameter steel ball and Va 11A brass disc spaced 0.1" from each other and provided with electrical connections through the cell wall to a 60 cycle alternating current source, so as to impose a variable voltage across the electrodes. The various mixtures represented on the drawing were determined -by pressure measurements. Por example, to prepare a NSP3-30% SP6 by volume mixture, the cell was evacuated of air, filled with NSP3 to 3.2 crn. Hg, and SFS was added to bring the final pressure of the gaseous mixture to Iabout 76 cm. Hg. Voltage across the electrodes was gradually increased until a spark jumped between the electrodes. The sparkover voltage was recorded as the breakdown voltage.

Referring to the drawing, there are illustrated 60 cycle breakdown voltages for different NSPa-SPG mixtures at atmospheric pressure in a non-uniform iield, described above.

It will be observed from the drawing that as the percent of NSF3 in the NSE-SP6 mixture increases, the dielectric strength of the mixture substantially increases over the strength of pure SP6. The most practical range of NSP3 in the mixture is between 10% and 80%. Below 10% NSP3 the dielectric strength increase over pure SP6 is minimal. If electrical breakdown occurs in the dielectric medium containing above 80% NSP3, extensive chemical decomposition results, accompanied by emission of light and heat. Electrical breakdown occurs, for example, when there is a momentary surge in the electric `potential of the electrodes of an apparatus, thus exceeding the strength of the dielectric. Such decomposition is undesirable in electrical apparatus. Decomposition upon electrical .breakdown does not occur when mixtures of less than 80% NSP3 are used. The optimum range of composition of the subject mixture has been found to be 25 to 75% NSF3 and, correspondingly, 75 to 25% SP6. This range provides the greatest increase in dielectric strength of the mixture over pure SP6, as well as a margin of safety with respect to chemical decomposition of the NSP3.

Since pure NSP3 cannot be used due to the abovementioned chemical decomposition, it was indeed unexpected that mixtures of NSP3 and SP6 could be successfully employfed as dielectric media hav-ing improved dielectric strength.

If desired, the gaseous mixtures of this invention may be employed under pressure with improvements in dielectric strength, over those obtained at one atmosphere, similar in magnitude to the improvements obtained when SP6 is used under pressure.

While the invention has been described with particular reference to speciiic embodiments, it is to be understood that it is not to be limited thereto but is to be construed broadly and restricted solely by the scope of the appended claims.

I claim:

1. A gaseous electrical insulating medium comprising a mixture of thiazyltriuoride and sulfur hexauoride, wherein the thiazyltriliuoride is present in an 4 amount suicient to increase the dielectric strength of ileferences Cited the mixture over the dielectric strength of pure sulfur `WOREIGN PATENTS hexauoride. w

2. The gaseous electrical insulating medium or claim 340,524 1T/1962 Canada 1 wherein said mixture comprises 10 to 80% by volume wjjTHER REFERENCES 3. The gaseous electrical insulating medium or claim 1 wherein said mixture contains 25 to 75% by volume LEON D' ROSDOL Pnmary Exammer of thiazyltriuoride. l". D. WALSH, `Assistant Examiner. 

